The Justice Department is investigating Uber's alleged use of "Greyball" software to circumvent government regulators.

2017 has not been a good year for Uber. While the company got a reprieve of sorts this week in its ongoing legal battle with Waymo over self-driving car tech, it now faces a Justice Department criminal investigation.

The Justice Department recently opened a criminal probe into Uber's "Greyball" software, according to The Washington Post, citing an anonymous source familiar with the matter. Existence of the software, which was allegedly used to circumvent government regulators, as made public earlier this year.

According to reports so far, the "Greyball" program was used to help Uber evade government regulators in cities where it was not officially approved to operate. It identified government officials trying to hail rides in order to prove that Uber was operating illegally in a given city, then blocked those rides.

The software has reportedly been present in the Uber app since 2014. It uses data on usage patterns to determine whether a person should be "Greyballed," and also looks at payment information to see if it is affiliated with any specific organizations.

Uber previously said the software was designed to catch riders who violated its terms of service, including "people aiming to physically harm drivers, competitors looking to disrupt our operations, or opponents who collude with officials on secret 'stings' meant to entrap drivers."

In a letter dispatched last week, Uber told Portland authorities, who had requested information on the program as part of a civil probe, that "Greyball" refers to showing certain users a different version of the app, according to The Washington Post. Uber said the program was used "exceedingly sparingly" in Portland and hasn't been used since April 2015, when the city adopted preliminary rules allowing Uber to operate.